Optical Molecular weighing scale with zeptogram sentivity
International Conference on Energy, Materials and Photonics EMP18, 8-11 July 2018, Montréal, Canada. 2018.
An ideal molecular weighing scale would be a mass sensor capable of weighing minute amounts of matter, down to a single molecule. The most sensitive mass sensors are based on nanomechanical oscillators: mass sensing of 7 zeptograms (zg) has been demonstrated using a microcantilever, and more recently a yoctogram sensitivity has been demonstrated using an oscillating carbone nanotube as the sensing unit. However, these impressive results are based on delicate measurements performed both in high vacuum and at cryogenic temperatures. We propose an optical mass sensor based on a hybrid photonic-plasmonic approach. We report on the fabrication of plasmonic hybrid sensors made of a micro-axicon dielectric lens and a single metallic nanoparticle. The dielectric lens produces a Bessel-like beam of light at its apex under plane wave excitation confining light in a narrow and intense beam with a low divergence. This Bessel-like beam can be used to optimize efficiently the excitation of a plasmonic nano-object and the collection of its far-field collected signal. We use a gold nanoparticle sitting onto a tapered glass waveguide as a weighing scale, transducing the mass into an optical signal. This concept can be seen as the optical analogue of the aforementioned mechanical mass sensors: instead of the resonance frequency of a mechanical oscillator, we monitor the resonance frequency of a charge oscillation, i.e., the localized surface plasmon (LSP) sustained by the gold nanoparticle. This optical mass sensor works at room temperature and atmospheric pressure, and enables the detection of mass inside a fluid with a sensitivity of a few tens of zg. Fig. 1. (a) Schematic representation of the developed system. (b) AFM image showing one axicon lens functionalized with a single gold nanoparticle. (c) Limit of detection measured onto a dimer for both polarizations (longitudinal in red and transverse in blue). Acknowledgements, Financial supports from the ANR “grants SMFLUONA, NATO”, the European community (FEDER funds), the « conseil général de l’Aube » and the « Conseil Régional Grand-Est», grant NANO'MAT (www.nanomat.eu) are highly acknowledged.